This invention generally relates to a process for converting N-substituted N-(phosphonomethyl)glycines (sometimes referred to as "N-substituted glyphosate"), as well as esters and salts thereof, to N-(phosphonomethyl)glycine (sometimes referred to as "glyphosate"), as well as esters and salts thereof, via a noble-metal catalyzed oxidation reaction. This invention is particularly directed to converting N-substituted glyphosates, as well as esters and salts thereof, having a single N-carboxymethyl functionality.
Glyphosate is described by Franz in U.S. Pat. No. 3,799,758 and has the following formula: ##STR3## Glyphosate and its salts conveniently are applied as a post-emergent herbicide in an aqueous formulation. It is a highly effective and commercially important broad-spectrum herbicide useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making glyphosate from N-substituted glyphosates are known in the art. For example, in U.S. Pat. No. 3,956,370, Parry et al. teach that N-benzylglycine may be phosphonomethylated to N-benzyl glyphosate, and then reacted with hydrobromic or hydroiodic acid to cleave the benzyl group and thereby produce glyphosate. In U.S. Pat. No. 3,927,080, Gaertner teaches that N-t-butylglycine may be phosphonomethylated to form N-t-butyl glyphosate, and then converted to glyphosate via acid hydrolysis. Glyphosate also may be produced from N-benzyl glyphosate via hydrogenolysis, as described, for example, in European Patent Application No. 55,695 and Maier, L. Phosphorus, Sulfur and Silicon, 61, 65-7 (1991). These processes are problematic in that they produce undesirable byproducts such as isobutylene and toluene which create difficulties due to their potential toxicities. Moreover, acid hydrolysis and hydrogenation of N-substituted glyphosates has been demonstrated only for alkyl groups such as tertiary butyl and benzyl groups which are known to be susceptible to such reactions. Dealkylation of N-methyl, N-isopropyl, and other N-substituted glyphosates which are not readily susceptible to acid hydrolysis or catalytic hydrogenation has not been demonstrated.
Other methods for making glyphosate are directed to oxidatively cleaving N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as "PMIDA"): ##STR4## PMIDA may be synthesized from phosphorus trichloride, formaldehyde, and an aqueous solution of the disodium salt of iminodiacetic acid, as described by Gentilcore in U.S. Pat. No. 4,775,498: ##STR5## It is well-known in the art that PMIDA may be converted into glyphosate by heterogeneous oxidation over carbon catalysts as described, for example, in U.S. Pat. No. 3,950,402 to Franz and U.S. Pat. No. 4,654,429 to Balthazor et al.; by homogenous catalytic oxidation as described, for example, in Riley et al. J. Amer. Chem. Soc. 113, 3371-78 (1991) and Riley et al. Inorg. Chem. 30, 4191-97 (1991); and by electrochemical oxidation using carbon electrodes as described, for example, in U.S. Pat. No. 3,835,000 to Frazier et al. These oxidation methods, however, have been reported to be useful only for preparing glyphosate from PMIDA, an N-substituted glyphosate having two N-carboxymethyl functionalities. None of these prior art oxidation methods have been reported to be useful for preparing glyphosate from N-substituted glyphosate compounds having only one N-carboxymethyl functionality, i.e., where R' in the following formula is other than --CH.sub.2 CO.sub.2 H: ##STR6## To the contrary, many prior art references suggest that if R' is a functionality other than a --CH.sub.2 CO.sub.2 H group, the prior art methods will cleave the --CH.sub.2 CO.sub.2 H group rather than R', and will therefore fail to produce glyphosate. This is particularly true for the prior art which is directed to heterogenous catalytic oxidations over carbon and electrochemical oxidations using carbon electrodes. The mechanisms for these oxidations are well known in the art, particularly for electrochemical oxidations where it is known as the Kolbe reaction, described in various organic electrochemistry books, e.g., S. Torii and H. Tanaka, Organic Electrochemistry 535-80 (H. Lund and M. M. Baizer eds., Marcel Dekker, 3rd ed. 1991). Both mechanisms involve the oxidative degradation of carboxylic acid to a carbon radical and carbon dioxide: ##STR7## There is no suggestion that these mechanisms could be used to cleave any other functionality besides --CH.sub.2 CO.sub.2 H.
Thus, a more general method for oxidizing N-substituted glyphosates to glyphosates is therefore desirable. Such a method would allow a wider range of N-substituted glycines to be used as raw materials for the production of glyphosate. Such a method also could be used to make glyphosate from N-methylglyphosate (sometimes referred to as "NMG"), an undesirable byproduct from the carbon-catalyzed oxidation of PMIDA.